Apparatus and method for histogram stretching

Abstract

A graphics controller including a memory and a contrast adjustment module is provided to perform on-board histogram stretching. The memory is configured to store a histogram table for a digital image. The contrast adjustment module is configured to read the histogram table stored in the memory and compute a histogram stretching transfer function based on the read histogram table. The contrast adjustment module is further configured to apply the computed histogram stretching transfer function to digital image data to be subsequently received by the contrast adjustment module.

1 . A graphics controller, comprising: 
 a memory configured to store a histogram table for a digital image; and    a contrast adjustment module configured to read the histogram table stored in the memory, the contrast adjustment module being further configured to compute a histogram stretching transfer function based on the read histogram table and apply the computed histogram stretching transfer function to digital image data to be subsequently received by the contrast adjustment module.    
   
   
       2 . A graphics controller as recited in  claim 1 , wherein the contrast adjustment module is configured to compute and apply the histogram stretching function without involvement of a processor of a host device of the graphics controller.  
   
   
       3 . A graphics controller as recited in  claim 1 , wherein the contrast adjustment module is configured to compute the histogram stretching transfer function without reading an entirety of the histogram table stored in the memory.  
   
   
       4 . A graphics controller as recited in  claim 1 , wherein the contrast adjustment module is configured to compute and apply the histogram stretching transfer function on a pixel-by-pixel basis to each pixel represented within the digital image data to be subsequently received by the contrast adjustment module.  
   
   
       5 . A graphics controller as recited in  claim 1 , wherein the histogram stretching transfer function is computed and applied by the contrast adjustment module without requiring storage of the computed histogram stretching transfer function prior to application of the computed histogram stretching transfer function.  
   
   
       6 . A graphics controller as recited in  claim 1 , further comprising: 
 a camera interface configured to transmit incoming digital image data to the contrast adjustment module; and    an image portion of the memory configured to receive digital image data from the contrast adjustment module and store the digital image data received from the contrast adjustment module,    wherein the contrast adjustment module is configured to compute and apply the computed histogram stretching transfer function to the digital image data during transmission of the digital image data from the camera interface through the contrast adjustment module to the image portion of the memory.    
   
   
       7 . A contrast adjustment module of a graphics controller, comprising: 
 a stretching point computation module configured to compute a first stretching point and a second stretching point to be used in performing a histogram stretching operation, each of the first and second stretching points corresponding to a respective pixel value;    a transfer function computation module configured to compute a portion of a histogram stretching transfer function based on the first and second stretching points and digital image data to be received by the contrast adjustment module;    a multiplexer configured to output a contrast adjusted pixel value according to a select signal to be received by the multiplexer, the multiplexer being further configured to receive as an input the portion of the histogram stretching transfer function as computed by the transfer function computation module; and    a comparison module configured to generate the select signal for the multiplexer, the select signal being generated based on the first and second stretching points and the digital image data to be received by the contrast adjustment module.    
   
   
       8 . A contrast adjustment module of a graphics controller as recited in  claim 7 , wherein the first stretching point is computed based on a number of pixels in the digital image to be allowed between a minimum possible pixel value and the first stretching point, the second stretching point being computed based on a number of pixels in the digital image to be allowed between the second stretching point and a maximum possible pixel value.  
   
   
       9 . A contrast adjustment module of a graphics controller as recited in  claim 8 , wherein the contrast adjustment module is connected to read values stored in a first register and a second register, the first register allocated to store the number of pixels in the digital image to be allowed between the minimum possible pixel value and the first stretching point, the second register allocated to store the number of pixels in the digital image to be allowed between the second stretching point and a maximum possible pixel value.  
   
   
       10 . A contrast adjustment module of a graphics controller as recited in  claim 9 , wherein values stored in the first and second registers are set such that a sum of the number of pixels in the digital image between the minimum possible pixel value and the first stretching point and between the second stretching point and a maximum possible pixel value corresponds to 10% of a total number of pixels in the digital image.  
   
   
       11 . A contrast adjustment module of a graphics controller as recited in  claim 7 , wherein the portion of the histogram stretching function corresponds to pixel values greater than or equal to the first stretching point and less than or equal to the second stretching point.  
   
   
       12 . A contrast adjustment module of a graphics controller as recited in  claim 7 , wherein the portion of the histogram stretching function is defined as  
     
       
         
           
             
               
                 MaxPixelValue 
                 ⁡ 
                 
                   ( 
                   
                     InputPixelValue 
                     - 
                     a 
                   
                   ) 
                 
               
               
                 b 
                 - 
                 a 
               
             
             , 
           
         
       
       wherein MaxPixelValue is the maximum possible pixel value, InputPixelValue is a pixel value within the digital image data to be received by the contrast adjustment module, a is the pixel value corresponding to the first stretching point, and b is the pixel value corresponding to the second stretching point.  
     
   
   
       13 . A contrast adjustment module of a graphics controller as recited in  claim 7 , wherein the contrast adjusted pixel value output by the multiplexer corresponds to either the minimum possible pixel value, the portion of the histogram stretching transfer function as computed by the transfer function computation module, or the maximum possible pixel value.  
   
   
       14 . A contrast adjustment module of a graphics controller as recited in  claim 7 , wherein the select signal is defined to direct the multiplexer to output the minimum possible pixel value when a pixel value within the digital image data is less than the first stretching point, the select signal being defined to direct the multiplexer to output the portion of the histogram stretching transfer function as computed by the transfer function computation module when the pixel value within the digital image is greater than or equal to the first stretching point and less than or equal to the second stretching point, the select signal being defined to direct the multiplexer to output the maximum possible pixel value when the pixel value within the digital image is greater than the second stretching point.  
   
   
       15 . A method for operating a graphics controller to adjust contrast of a digital image received from a digital imaging device, comprising: 
 operating a contrast adjustment module within the graphics controller to read a histogram table stored in a memory of the graphics controller;    operating the contrast adjustment module to compute a histogram stretching transfer function to be applied to digital image data received from the digital imaging device, the histogram stretching function being based in part on the histogram table read from the memory;    receiving the digital image data at the contrast adjustment module; and    operating the contrast adjustment module to apply the histogram stretching transfer function to the received digital image data, wherein application of the histogram stretching transfer function to the received digital image data serves to adjust a contrast of a digital image defined by the received digital image data.    
   
   
       16 . A method for operating a graphics controller to adjust contrast of a digital image to be received from a digital imaging device as recited in  claim 15 , wherein operating the contrast adjustment module to compute the histogram stretching transfer function includes calculating a first stretching point and a second stretching point, each of the first and second stretching points corresponding to a respective pixel value.  
   
   
       17 . A method for operating a graphics controller to adjust contrast of a digital image to be received from a digital imaging device as recited in  claim 16 , wherein the first stretching point is computed based on a number of pixels in the digital image to be allowed between a minimum possible pixel value and the first stretching point, the second stretching point being computed based on a number of pixels in the digital image to be allowed between the second stretching point and a maximum possible pixel value.  
   
   
       18 . A method for operating a graphics controller to adjust contrast of a digital image to be received from a digital imaging device as recited in  claim 16 , wherein operating the contrast adjustment module to compute the histogram stretching transfer function includes determining a histogram stretching transfer function value corresponding to a pixel value greater than or equal to the first stretching point and less than or equal to the second stretching point.  
   
   
       19 . A method for operating a graphics controller to adjust contrast of a digital image to be received from a digital imaging device as recited in  claim 18 , wherein the histogram stretching transfer function value corresponding to the pixel value greater than or equal to the first stretching point and less than or equal to the second stretching point is defined as  
     
       
         
           
             
               
                 MaxPixelValue 
                 ⁡ 
                 
                   ( 
                   
                     InputPixelValue 
                     - 
                     a 
                   
                   ) 
                 
               
               
                 b 
                 - 
                 a 
               
             
             , 
           
         
       
       wherein MaxPixelValue is a maximum possible pixel value, InputPixelValue is a pixel value within the digital image data received by the contrast adjustment module, a is the pixel value corresponding to the first stretching point, and b is the pixel value corresponding to the second stretching point.  
     
   
   
       20 . A method for operating a graphics controller to adjust contrast of a digital image to be received from a digital imaging device as recited in  claim 16 , wherein operating the contrast adjustment module to compute the histogram stretching transfer function includes comparing the pixel value within the digital image data received by the contrast adjustment module to the first and second stretching points to determine an appropriate histogram stretching transfer function value to be output by the contrast adjustment module in place of the received pixel value.